Effect of large amounts of natural pozzolan addition on properties of blended cements

In this study, the effects of 35, 45, and 55 wt.% natural pozzolan addition on the properties of blended cement pastes and mortars were investigated. Blended cements with 450 m²/kg Blaine fineness were produced from a Turkish volcanic tuff in a laboratory mill by intergrinding portland cement clinker, natural pozzolan, and gypsum. The cements were tested for particle size distribution, setting time, heat of hydration, compressive strength, alkali-silica activity, and sulfate resistance. Cement pastes were tested by TGA for Ca(OH)₂ content and by XRD for the crystalline hydration products. The compressive strength of the mortars made with blended cements containing large amounts of natural pozzolan was lower than that of the portland cement at all tested ages up to 91 days. Blended cements containing large amounts of pozzolan exhibited much less expansion with respect to portland cement in accelerated alkali-silica test and in a 36-week sulfate immersion test.     

Assessment of phase formation in lime-based mortars with added metakaolin, Portland cement and sepiolite, for grouting of historic masonry

Lime-based mortars containing pozzolanic additions of metakaolin, sepiolite and white Portland cement are studied in order to determine their performance as historic masonry conservation mortars. Hydration products on metakaolin-lime blended mortars include stable and metastable phases. The presence of such products has been studied by means of DTA and XRD analysis, concluding that the selection between them is mainly related with the water-lime ratio. Sepiolite addition to metakaolin-lime mortars has shown to inhibit C₄AH₁₃ formation. Therefore, the influence of phase distribution on the mechanical resistance is considered. Finally, compounds production on blended lime-white Portland cement was compared to natural hydraulic lime ones, and as a result, no remarkable differences appeared, apart from traces of possible cement Portland addition to the latter, usually not mentioned in the nominal composition supplied by the manufacturers of lime binders.     

Experimental studies and thermodynamic modeling of the carbonation of Portland cement,metakaolin and limestone mortars

The carbonation of Portland cement, metakaolin and limestone mortars has been investigated after hydration for 91 days and exposure to 1% (v/v) CO₂ at 20 °C/57% RH for 280 days. The carbonation depths have been measured by phenolphthalein whereas mercury intrusion porosimetry (MIP), TGA and thermodynamic modeling have been used to study pore structure, CO₂ binding capacity and phase assemblages. The Portland cement has the highest resistance to carbonation due to its highest CO₂ binding capacity. The limestone blend has higher CO₂ binding capacity than the metakaolin blends, whereas the better carbonation resistance of the metakaolin blends is related to their finer pore structure and lower total porosity, since the finer pores favor capillary condensation. MIP shows a coarsening of the pore threshold upon carbonation for all mortars. Overall, the CO₂ binding capacity, porosity and capillary condensation are found to be the decisive parameters governing the carbonation rate.     

Effect of silicate modulus and metakaolin incorporation on the carbonation of alkali silicate-activated slags

Accelerated carbonation is induced in pastes and mortars produced from alkali silicate-activated granulated blast furnace slag (GBFS)-metakaolin (MK) blends, by exposure to CO₂-rich gas atmospheres. Uncarbonated specimens show compressive strengths of up to 63 MPa after 28 days of curing when GBFS is used as the sole binder, and this decreases by 40-50% upon complete carbonation. The final strength of carbonated samples is largely independent of the extent of metakaolin incorporation up to 20%. Increasing the metakaolin content of the binder leads to a reduction in mechanical strength, more rapid carbonation, and an increase in capillary sorptivity. A higher susceptibility to carbonation is identified when activation is carried out with a lower solution modulus (SiO₂/Na₂O ratio) in metakaolin-free samples, but this trend is reversed when metakaolin is added due to the formation of secondary aluminosilicate phases. High-energy synchrotron X-ray diffractometry of uncarbonated paste samples shows that the main reaction products in alkali-activated GBFS/MK blends are C-S-H gels, and aluminosilicates with a zeolitic (gismondine) structure. The main crystalline carbonation products are calcite in all samples and trona only in samples containing no metakaolin, with carbonation taking place in the C-S-H gels of all samples, and involving the free Na⁺ present in the pore solution of the metakaolin-free samples. Samples containing metakaolin do not appear to have the same availability of Na⁺ for carbonation, indicating that this is more effectively bound in the presence of a secondary aluminosilicate gel phase. It is clear that claims of exceptional carbonation resistance in alkali-activated binders are not universally true, but by developing a fuller mechanistic understanding of this process, it will certainly be possible to improve performance in this area.     

Cement substitution by a combination of metakaolin and limestone

This study investigates the coupled substitution of metakaolin and limestone in Portland cement (PC). The mechanical properties were studied in mortars and the microstructural development in pastes by X-ray diffraction, thermogravimetry analysis, mercury intrusion porosimetry and isothermal calorimetry. We show that 45% of substitution by 30% of metakaolin and 15% of limestone gives better mechanical properties at 7 and 28 days than the 100% PC reference. Our results show that calcium carbonate reacts with alumina from the metakaolin, forming supplementary AFm phases and stabilizing ettringite. Using simple mass balance calculations derived from thermogravimetry results, we also present the thermodynamic simulation for the system, which agrees fairly well with the experimental observations.It is shown that gypsum addition should be carefully balanced when using calcined clays because it considerably influences the early age strength by controlling the very rapid reaction of aluminates.     

The effect of pozzolans and slag on the expansion of mortars cured at elevated temperature: Part II: Microstructural and microchemical investigations

The microstructural and microchemical development of heat-cured Portland cement mortars containing silica fume, metakaolin, blast-furnace slag, and fly ash were analysed using pore solution analysis, X-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy-dispersive X-ray analysis (EDX). Incorporation of these materials into the mixture modifies the composition of the C-S-H gel, the quantities of the hydration products, and the microstructure. Ettringite was formed during moist storage in all specimens, but was not accompanied by expansion where a sufficient amount of metakaolin, blast-furnace slag, or a suitable fly ash replaced a proportion of the Portland cement; replacement with silica fume was not as effective at eliminating expansion. The different behaviour of silica fume from the other supplementary cementing materials is believed to reflect a difference in the way ettringite is formed in the presence of Al₂O₃-bearing mineral admixtures.     

Exploitation of poor Greek kaolins: strength development of metakaolin concrete and evaluation by means of k-value

In this paper, the effect of metakaolin on concrete properties is investigated. A poor Greek kaolin was thermally treated at defined conditions, and the produced metakaolin was superfine ground. In addition, a commercial metakaolin of high purity was used. Eight mixture proportions were used to produce high-performance concrete, where metakaolin replaced either cement or sand in percentages of 10% or 20% by weight of the control cement content. The strength development of metakaolin concrete was evaluated using the efficiency factor (k-value). The produced metakaolin as well as the commercial one imparts a similar behavior with respect to the concrete strength. Both metakaolins exhibit very high k-values (close to 3.0 at 28 days) and are characterised as highly reactive pozzolanic materials that can lead to concrete production with an excellent performance.     

The effect of SCMs and curing time on resistance of mortars subjected to organic acids

Agricultural effluents such as liquid manure and ensilage effluents contain organic acids that constitute a severe chemical threat toward the concrete of agricultural structures. In contact with an acidic solution, the chemical equilibrium of the hydrates in cement paste is destabilized, causing negative effects on porosity, reinforcement corrosion, mechanical strength, and, in the long term, may result in the collapse of the structure. More durable concrete in this environment is needed. The purpose of this study is to examine the effect of the nature of the supplementary cementing materials (SCMs) as well as the curing time on the chemical and the physical modifications of cement pastes and on the compressive strength, mass loss, altered depth and microstructure of mortars immersed in acetic acid at a pH of 4.This study concentrated on three types of hardened cement pastes or mortars made with ordinary Portland cement (OPC), slag and metakaolin cements, cured for period varying from 28 days to 1 year. The results show the beneficial effect of the curing time before the acid immersion, the better durability of metakaolin cement and the good chemical resistance of the slag cement against acid attack. The latter develops low compressive strength and is more sensitive to the curing time but the drop of its resistance due to the acid immersion is minimal due to its strong chemical resistance.     

Influence of spent catalyst used for catalytic cracking in a fluidized bed on sulphate corrosion of cement mortars: I. Na2SO4 medium

The aim of the work was to determine the effect of spent catalytic cracking in a fluidized bed (FBCC), catalyst used for as a partial substitute for cement or sand, and of sodium sulphate solutions of concentration 16,000 or 52,000 mg SO₄²⁻/dm³ on the mechanism of sulphate corrosion and on expansion of mortars compacted according to prENV 196-10, as well as on mechanical strength of mortars compacted according to the Polish Standard PN-EN 196-1. The observations indicate that the sulphate corrosion of the mortars proceeds via at least two different mechanisms depending on the concentration of sodium sulphate solution and on the concentration and form of pozzolanic additive used. The extent of corrosion damage depends mainly on the concentration of the aggressive solution and on the degree of compaction of the mortar. No unequivocal correlation was found between the expansion of mortars compacted according to prENV 196-10 and the mechanical strength of mortars compacted according to PN-EN 196-1. The greatest expansion was observed in mortars, which did not exhibit the lowest compressive strength. Therefore, it is questionable to use the measure of elongation, particularly in poorly compacted mortar samples, as the sole standard method for determining resistance to sulphate corrosion in sodium sulphate media.     

Aluminophosphate bonded refractory mortars

Conclusions Chamotte mortars containing aluminophosphate bonds considerably improve the strength of the joint between firebrick refractories after heating to 400–800°C compared with mortars based on Portland cement and water glass. The highest strength for the mortar-firebrick bond is provided with a phosphoric acid density of 1.42–1.43 g/cm³ (60% phosphoric acid), and incorporating 20% bond in the mortar.It is possible to increase the strength of the chamotte mortar containing aluminophosphate bond during air setting and during heating, and also to increase the strength with time, by adding active materials to the mortar.Aluminophosphate-bonded mortar markedly increases the strength of the refractory-metal bonding, which is very important in building linings which are subject to movement during operation for example, in rotary kilns. It is desirable to test mortars containing aluminophosphate bonds in rotary kilns, and also in similar plant in the refractories, ceramic, and cement industries.Translated from Ogneupory No. 1, pp. 37–42, January, 1971.     

Corrosion resistance and chloride diffusivity of volcanic ash blended cement mortar

This article reports the results of an investigation on the chloride diffusivity and corrosion resistance of volcanic ash (VA) blended cement mortars with varying curing times of up to 1 year. The mortars had 20% and 40% VA as cement replacement and water/binder ratio of 0.55. The accelerated chloride ion diffusion (ACID) test was used to calculate the chloride ion (Cl⁻) diffusion coefficient (D_i) of the mortars using the Nernst-Plank equation for steady state condition. In addition, electrical resistivity, mercury intrusion porosimetry and differential scanning calorimetry (DSC) tests were conducted. Electrochemical measurement such as linear polarization resistance was used to monitor the corrosive behavior of the embedded steel bars. The chloride ingress into the mortars was also studied. Good correlations were found among D_i, total pore volume (TPV) and electrical resistivity of the mortars. It was also found that blending cement with VA significantly reduced the long-term D_i and hence increased the long-term corrosion resistance of mortars. This fact was also supported by the presence of lower quantity of Ca(OH)₂ and higher quantity of Friedel's salt in the VA blended mortars as observed from the DSC tests. Mortars with 40% VA showed better performance in terms of Cl⁻ diffusivity, chloride ingress and passivation period of embedded steel compared with the control mortar with 0% VA.     

High-slag cement mortars for brickwork and renderings

The suitability of clinker-activated high-slag cement mortars for use in brickwork construction and in rendered finishes, has been investigated. The study was carried out on a slag cement composed of 80% granulated slag, 15% clinker and 5% raw gypsum; its specific surface area was 4000 cm²/g. It was found that the slag cement mortars and cement-lime mortars of the proportions commonly used in practice, have higher water-retentivity than the corresponding mortars of ordinary Portland cement, and that their strengths are comparable to those of the latter mortars.     

Pozzolanic reactions of six principal clay minerals: Activation, reactivity assessments and technological effects

Six standard clays, before and after calcination at 3 or 4 temperatures and being mixed with Ca(OH)₂ [CH] in the presence of simulated cement pore solution, and with ordinary Portland cement, respectively, were studied in detail. Chemical compositions of most clays conform well to the requirement in ASTM C 618. Water demand of clay-containing mortar varies, depending on the crystal chemistry of raw clays, and on the specific surface area of calcined clays. Measurements of XRD background or alkali soluble Si are rapid methods in evaluation of the pozzolanic activity of clays. Compressive strength of mortars based on the raw clays is affected by structure of clays. Calcination increases the pozzolanic activity of clays and the compressive strength of the Portland cement — clay mortars. A close correlation exists between compressive strength of mortars and particle size distribution of the dehydroxylated clays. The most common reaction products of clay — CH mixtures are C-S-H² and C₄AH_x, while C₂ASH_j8 and C₃AH₆ were also detected with clays rich in Al.     

Assessment of mechanical and microstructural properties of geopolymers produced from metakaolin,silica fume,and red mud

Geopolymers have been studied as viable alternative to traditional Portland cement-based products, given the use of industrial by-products as raw materials. This work evaluated the mechanical and microstructural properties of geopolymeric mortars produced with sodium hydroxide solution, metakaolin, silica fume, and red mud. The mixtures were produced by means of dosages with different molar ratios and curing conditions. The raw materials were characterized by granulometry, chemical, mineralogical, and thermal analysis. The characterization of mortars was performed by scanning electron microscopy (SEM) and axial compressive strength tests. The precalcination at 850°C of the red mud was sufficient to make it more reactive and suitable for use in geopolymers. Noteworthy, the best mechanical strengths of metakaolin mortars for curing at 50°C, and with the lowest SiO₂/Al₂O₃ ratios. In the mortars with incorporated red mud, there was a decrease of strength at thermal curing conditions and with the increase of residue content, whose microstructure indicates the formation of more pores in the geopolymer matrix. The thermal curing promoted the formation of sodalite crystals, and the significant presence of Na particles on the surface suggests that part of the added NaOH did not react with the precursors.     

Effect of waste aluminosilicate material on cement hydration and properties of cement mortars

The effect of waste material (catalyst used previously in catalytic cracking of petroleum in fluidized bed—fluidized bed cracking catalyst denoted as FBCC) on cement hydration kinetics was investigated in terms of fineness of this admixture. The compressive strength and microstructure of cement mortars were also examined. Variable percentage of this aluminosilicate admixture, originating from batches of quite different grain size composition, was introduced to cement pastes. Further on, cement mortars were produced using the material of higher activity, as it has been found in admixtured cement investigations. The waste was added as cement replacement or, partially, as sand replacement. The activity of waste catalyst was strongly related to the fineness—finer grains indicate better activity. In the presence of a FBCC admixture, the Ca(OH)₂ content decrease in cement pastes due to the pozzolanic reaction is observed. The surface area of hydrated paste becomes higher and, simultaneously, the mean pore diameter decreases, as compared to reference sample, without admixture. The strength improvement is observed particularly when the aluminosilicate material is introduced as partial sand replacement.     

Physical performances of alkali‐activated portland cement‐glass‐limestone blends

The potential of calcium aluminosilicate (CAS) glasses as supplementary cementitious materials is studied in terms of the development of compressive strength for mortars containing a mixture of portland cement, CAS glass, and limestone. In addition, the impact of internal and external alkali activation of the cementitious systems on the mortar performances is investigated. Internal alkali activation is obtained by adding alkali oxides to the CAS glass system, whereas external alkali activation is realized by hydration of the blended cements containing alkali‐free CAS glasses using alkaline solutions. For the internally alkali‐activated systems and the alkali‐free mortars, higher strengths are achieved in comparison to the reference mortar prepared from plain ordinary portland cement. In contrast, the externally alkali‐activated mortars exhibit lower compressive strengths, implying the importance of both the immediate availability of alkali ions in the cementitious system and the increased dissolution rate of the glass particles caused by the network depolymerization. The glasses are also studied by thermal analysis and the results are used to calculate the theoretical CO₂ emissions. The lowest embodied CO₂ emission is estimated for the blends containing alkali‐activated CAS glasses.     

Effects of an amphoteric copolymer on the properties of cementitious materials

The effects of an amphoteric copolymer, poly(acrylamide‐co‐(2‐(3‐carboxyacryloyloxy)‐N‐(carboxymethyl)‐N,N‐dimethyl‐ethanaminium) (PAC) on the properties of cement pastes or mortars were investigated. PAC was prepared from acrylamide and 2‐(3‐carboxy‐acryloyloxy)‐N‐(carboxymethyl)‐N,N‐dimethylethanaminium (CAC) through free radical polymerization. The results indicate that the presence of the polymer delayed the setting of cement pastes; it also reduced the water demand so that the resulting mortars showed improved compressive strength and reduced length change. PAC with about 16.7% CAC and a weight‐average molecular weight of about 5 × 10⁴ was effective in promoting the material properties of mortars. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Investigations on the performance of silica fume-incorporated cement pastes and mortars

Studies on the performance of cementitious products with silica fume (SF) are very important, as it is one of the inevitable additives to produce high-performance concrete (HPC). In this study, some experimental investigations on the influence of SF on various preliminary properties of cement pastes and mortars are reported. The properties included specific gravity and normal consistency (NC) of cement and air content and workability of mortar with different SF contents. Pozzolanic and chemical reactions of SF have been studied on setting times, soundness and shrinkage of cement pastes. Further, strength developments in compression and tension in cement mortars have also been studied at various SF contents. SF was varied from 0% to 30% at a constant increment 2.5/5% by weight of cement. Test results show that the SF changes the behavior of cement pastes and mortars significantly. It has been observed that the water-binder (w/b) (cement+SF) ratio seemed to play an important role for the performance of the products with higher SF contents. NC, soundness and drying shrinkage of cement pastes and the strength of mortar increase as the SF content increases, while the initial setting times of cement pastes and the air content and workability of mortar decrease as the SF content increases. However, hardly any influence has been observed on the final setting times of cement pastes. The early age hydration reactions of C₃A and C₃S increase with the addition of SF. The optimum SF content ranges between 15% and 22%.     

Effects of metakaolin, water/binder ratio and interfacial transition zones on the microhardness of cement mortars

Variations in the microhardness of the hydrated cement matrix component of model mortars have been investigated as functions of the distance from the aggregate surfaces for specimens in which the binder was Portland cement or a blend of Portland cement and metakaolin.Microhardness measurements were made using a Knoop indenter at distances of up to 120 μm from the aggregate. The microhardnesses of the paste-aggregate interfacial transition zones (ITZs) were found to be between 14% and 22% lower than those of the corresponding bulk cement pastes at the lower water/binder ratios investigated, i.e. 0.4 and 0.5 for samples prepared with Portland cement and 0.4 for samples prepared with a binder comprising Portland cement and metakaolin.Metakaolin increased the mean microhardness of specimens prepared at the higher water/binder ratios of 0.5 and 0.6 by 13% and 54%, respectively.     

The effect of waste oil-cracking catalyst on the compressive strength of cement pastes and mortars

Epcat, one of the spent fluid catalytic cracking (FCC) catalysts from oil-cracking refineries, shows pozzolanic activity. In this study, pastes and mortars with Epcat were prepared and cured, and their compressive strengths after 3, 7 and 28 curing days were measured. The water/binder (W/B) ratios were 0.2, 0.25 and 0.3, and the replacement levels of cement by Epcat were 0, 5, 10 and 15 wt.%. Proper amount of superplasticizer was added into each mix to ensure similar workability.The results indicate that the presence of Epcat would increase the compressive strength of mortars substantially, but increase the compressive strength of the related pastes only slightly. Epcat mortars with W/B=0.25 show more strength-enhancing effect than those with W/B=0.3, and this effect increases with the catalyst content. Therefore, the mix (W/B=0.25) incorporated 15% Epcat exhibits the greatest compressive strength (92.3 MPa). For mortars with W/B=0.2, the strength-enhancing effect occurs only for those containing 5% catalyst; this effect becomes unclear when mixes containing 10% Epcat or more because high dosage of superplasticizer was added in obtaining proper workability and that affects the strength development. The improvement in the mechanical properties of mortars is attributed to the increase in the hydrated cement paste itself and, more importantly, improved bonds between the cement paste and aggregate.